Wireless base stations in a wireless communication system such as a mobile communication system, a trunk communication system, or a broadcasting system, transmit radio signals with relatively large power outputs. Each power amplifier used in a transmission device is expected to output signals that have linearity with respect to input power, but exhibits non-linear, distorted components in reality. Due to the non-linearity of each power amplifier, unnecessary signal components are contained in output signals of the power amplifier, resulting in a distorted output waveform. If the side lobe exceeds the tolerance value on a frequency spectrum plot due to the unnecessary signal components, the neighboring channels are disturbed. To counter the non-linearity in the power amplifier, not only a waveform rectifying operation using a filter device but also a digital predistortion technique should be additionally employed in a conventional operation.
FIG. 1 is a block diagram of an amplifier device 100 that performs distortion compensation by the above mentioned technique. The amplifier device 100 converts a transmission input signal that is a digital baseband signal into a predistortion signal, using distortion compensating parameters read out from a lookup table 120. The predistortion signal is then converted into an analog signal by a digital-to-analog converter 104. A modulator 106 performs a modulating operation using the input analog signal. The modulated signal is a signal in the 800 MHz band that is used by general cellular telephone systems, or a signal in the 2 GHz band of IMT 2000. The modulated signal is then amplified by a power amplifier 108, and is transmitted through an antenna (not shown).
Part of the transmission signal is supplied to a demodulator 112 via a distributor 110. The demodulator 112 performs a demodulating operation to output a demodulated signal. The demodulated signal is then converted into a digital signal by an analog-to-digital converter 114, and is input to one of the two input ends of an error detecting circuit 116. A delay signal that is formed by delaying a transmission input signal by a delay circuit 118 is input to the other input end of the error detecting circuit 116. The delay amount at the delay circuit 118 is equivalent to the delay caused in the transmission input signal passing through the distributor 110 and reaching the error detecting circuit 116. Based on the difference between the feedback signal and the delayed signal, the error detecting circuit 116 outputs an error signal. The feedback signal is a signal based on the amplified signal, but is attenuated by the distributor or the like. Ideally, the signal output from the delay circuit 118 is equal to the feedback signal output from the analog-to-digital converter 114. More specifically, the error detecting circuit 116 measures the difference between the signal output from the delay circuit 118 and the feedback signal, and then changes the delay amount at the delay circuit 118 so as to reduce the difference. The delay amount is fixed to the value with which the difference is minimized, and the distortion compensating parameters in the lookup table 120 are updated. Thus, the error at the error detecting circuit 116 is controlled to become smaller. The updated distortion compensating parameters are supplied to the distortion compensating circuit 102.
The distortion compensating parameters are used to change the gain (amplitude) and phase of a transmission input signal in advance (to provide the opposite distortion characteristics), so that a transmission output signal having suitable linearity for the transmission input signal can be obtained. The signal having the opposite distortion characteristics is distorted at the modulator 106, the power amplifier 108, or the like, but is finally output with the desired characteristics. In an operating state, the updating of the delay amount in the delay circuit is performed at longer intervals than the updating of the lookup table, so that each of the parameters can be properly updated.
Although the distortion compensating circuit 102 appears to process a single signal in the structure shown in FIG. 1, each of the two types of signals Ich and Qch is processed in a case where the modulator 106 performs quadrature modulation.
The amplifier device 100 shown in FIG. 1 includes a number of analog devices. Foe example, a power amplifier is generally formed with a number of stages of analog devices. Also, it is necessary to employ a number of devices (such as inductors and capacitors) to obtain a steep profile in the characteristics of a filter for removing unnecessary waves. In general, analog devices exhibit various characteristics, and there are changes in the characteristics due to temperature change, change in properties with time, or the like. The changes in the characteristics might change the signal propagation time or the delay amount (and the phase angle) in each signal path. In short, the period of time existing between the inputting of a transmission input signal to the amplifier device 100 and the outputting of a transmission output signal from the amplifier device 100 (and the period of time required for obtaining a feedback signal) is changed. In such a case, the amplifier device 100 detects the change in the delay amount, and adjusts the delay amount at the delay circuit 118, based on the difference between the feedback signal and the delayed signal. In this manner, the amplifier device 100 can output a distortion-compensated transmission signal, coping with changes in the delay amount due to changes in properties with time or the likes.
There is a wireless base station in which amplifier devices of the above described type are operated in parallel and outputs of the amplifier devices are combined. The combining of the outputs of the amplifier devices is performed to obtain a larger power output, for example. Each of the paths is also connected to an antenna, so that the amplified outputs can be spatially combined while the phase of each path is adjusted. In this manner, an antenna beam of the desired directivity can be formed (transmission diversity). In the case of combining the amplified outputs in such a manner, it is necessary to make the delay amounts (and the phases) of the paths precisely equal to one another.
However, when the delay amount in each signal path changes due to a change in the analog device characteristics with temperature and time, the delay amounts vary between a transmission input signal and a transmission output signal at each signal path. As a result, signals of different timing properties are combined, and a desired large output and a desired antenna beam pattern might not be obtained.
Japanese Unexamined Patent Publication Nos. 63-208330, 2001-189685, and 2001-345718 disclose conventional distortion compensating amplifier devices. However, none of those devices is designed to properly combine the amplified outputs of two or more signal paths.